The oligomerization of olefins to higher hydrocarbons is a known process carried out with a zeolite oligomerization catalyst in a high pressure fluid bed reactor operating at oligomerization conditions of temperature and pressure. It was not known that the key to operating such a process economically was the ability to limit the deposition of coke in the reactor, allowing the withdrawal of a small slipstream of catalyst (recirculated stream) from the reactor; and, using pressure-isolatable lock-hopper zones between the reactor and the regenerator, and again between the regenerator and the reactor. This invention teaches such a process carried out in a reactor operating above about 515 kPa (60 psig) in which process the improvement comprises, depressurizing (or `depressurizing`) and purging the recirculating (`recirc` for brevity) stream, stripping and regenerating it at less than about one-half the reactor operating pressure, and repressuring (`pressurizing`) it to a pressure sufficient to return it to the reactor, using lock-hopper zones. The terms `depressuring` and `pressurizing` are used hereinafter because they are phonetically distinct.
Lower olefins in the feed are converted to heavier hydrocarbons ("heavies") in a single-zone fluid-bed reactor, operating at pressure and temperature conditions at which no liquid hydrocarbon phase is present, hence referred to as a "non-liquid phase". This non-liquid phase may be the gas phase, or one which is "super-dense", referred to as such, because the pressure is so high as to maintain the hydrocarbons in the fluid bed under near-critical or supercritical conditions. The term "super-dense phase" is not to be confused with the common usage of the term "dense phase" as typically applied to a fluidized bed of catalyst, but refers to the mixture of hydrocarbons at or above P.sub.max (defined herebelow) in the reactor. The particular conditions of operation of our process depend upon whether the fluid bed operates with hydrocarbons in the gas phase; or, operates in the super-dense phase, either of which is referred to as non-liquid. Operation of the reactor in the non-liquid phase is a preferred embodiment of this invention.
Feed to the reactor is introduced at the bottom, in an amount sufficient to fluidize the bed; the recirc stream, containing used (mostly deactivated) catalyst is withdrawn from the reactor and stripped, then regenerated and returned to the fluid bed. The oligomerized product leaves from the top of the reactor so that the relative direction of flow of catalyst and feed is typical of that in a fluid bed operating in the turbulent regime.
When the reactor is used to oligomerize olefins to gasoline, it is referred to as a MOG (Mobil Olefin to Gasoline) reactor operating in the gasoline mode; when used to oligomerize olefins to distillate, the reactor is referred to as a MOD (Mobil Olefin to Distillate) reactor operating in the distillate mode; when used to oligomerize olefins to lubes, the reactor is referred to as a MOL (Mobil Olefin to Lubes) reactor operating in the lubes mode; and, the reactor is referred to as a MOGDL reactor when it may be used to discharge any of the foregoing functions.
Operation of the MOG reactor in the gas phase under sub-critical pressure, is described in greater detail in our U.S. Pat. No. 4,777,316 issued Oct. 11, 1988 and in copending application Ser. No. 197,543 of Johnson D. and Avidan A. filed on May 23, 1988, the disclosures of which are incorporated by reference thereto as if fully set forth herein. In the "gas-phase" embodiment of our invention, operation of the reactor ensures that the reaction occurs in the gas phase, at conditions of pressure and temperature which are (1) below both P.sub.max and T.sub.max ; or (2) below P.sub.max but above T.sub.max. In the "super-dense" embodiment of our invention, the operation of the reactor ensures that the reaction occurs in the super-dense phase requiring conditions of pressure and temperature which are (3) at or above both P.sub.max and T.sub.max, so that in all instances, the reaction occurs outside the phase envelope, as described in greater detail in our copending patent application Ser. No. 184,465 filed Apr. 20, 1988 the disclosure of which is incorporated by reference thereto as if fully set forth herein.
By sub-critical pressure we refer to a pressure below P.sub.cr ; and by "near-critical" conditions we refer to a pressure P.sub.max at or above which no liquid may be present, this pressure typically being not less than about 344.5 kPa (50 psia) below the critical pressure P.sub.cr. By sub-critical temperature we refer to a temperature below T.sub.cr but also below T.sub.max, at or above which no liquid may be present. The precise P.sub.max and T.sub.max for a particular feedstock will vary depending upon its composition. For a typical, predominantly C.sub.3 -C.sub.4 light gas, P.sub.max is about 4000 kPa (565 psig) and T.sub.max is about 132.degree. C. (270.degree. F.). By "super-critical" conditions we refer to conditions above P.sub.cr, T.sub.cr for the product, and outside the "envelope" of the phase diagram. By "the phase diagram", we refer in all cases to the phase diagram for all the hydrocarbons present in the reactor at any moment.
Since our process is an exothermic process, most economically practiced with the reactor operating continuously, it is necessary to regenerate the catalyst, either continuously or semi-continuously, in a continuous process. By "continuous process" we refer to the reactor operating continuously. In such a process, the regenerator may operate semicontinuously, or, continuously, as will be explained hereinafter. Because the reactor operates at relatively high pressure with the hydrocarbons in the non-liquid phase, a practical process presents a peculiar set of problems which are exacerbated when operation is at near-critical, and typically, super-critical pressure and temperature. This invention relates to an ingenious and unobvious solution to problems associated with the non-liquid phase, whether the gas phase, or, the super-dense phase, at a pressure often well above about 650 kPa (80 psig).
As one skilled in art will appreciate, the mass of hydrocarbons in the non-liquid phase associated with the withdrawn catalyst is high because of the high pressure, and one must strip this mass from the catalyst before it is oxidatively regenerated in a regenerator. Since a prerequisite for stripping is high volumetric flow of stripping gas, it is economically out of the question to strip with an inert gas at elevated pressures, especially when the reactor operates above P.sub.max which is typically above about 2410 kPa (335 psig), and particularly uneconomical at or above about 6300 kPa (900 psig). Practical stripping requires it be done at relatively low pressure to derive a double-barreled benefit, both from a decrease in pressure, and from a high volumetric flow of a suitably economical stripping medium such as steam, at relatively low pressure. The precise low pressure at which stripping is to be effected is determined by the economics of the process, but for obvious reasons, will be the highest pressure at which stripping is economical. Except that, though steam is most convenient and economical, the zeolite oligomerization catalyst is highly sensitive to a high partial pressure of steam, which high partial pressure tends to deactivate the catalyst. This consideration militates against using steam except at a suitably low partial pressure.
Stripping a stream of catalyst withdrawn from a fluidized bed of catalyst is routinely done in a fluid catalytic cracker (FCC) with a number of variations of old techniques taught for example in U.S. Pat. No. 2,688,195 (class 34/57) and U.S. Pat. No. 2,833,699 (class 208/150). In the latter, steam jet blasts catalyst as it enters the stripping zone, followed by contacting the catalyst as a dense-phase fluidized bed. How to drop the pressure in the stripping zone, whether by a factor of two or more, is not a technical issue in the prior art, since the operation of the FCC reactor is never substantially above atmospheric, being usually below about 50 psig. Moreover, there is generally no problem in the prior art, with respect to coping with a sudden, large change of pressure after the hydrocarbons are withdrawn, or with a transition of phases they might undergo. They are always in the vapor phase, well below either P.sub.cr or P.sub.max, and usually well above either T.sub.cr or T.sub.max.
Still further, the entire bed of catalyst in a FCC reactor is stripped several times within an hour so that the rate of withdrawal of catalyst for stripping and regeneration is at least 50% of the bed per hr. The volumetric flow of stripping steam required is determined by the flow rate of the withdrawn catalyst, not the pressure in the reactor, so that the mass flow of steam relative to the withdrawn catalyst is low, determined by the reactor pressure. Thus the reactor pressure is only an incidental, rather than a critical factor, in the cost of stripping steam. Since in the teaching of the prior art there is no serious economic penalty related to the volumetric and mass flow of stripping steam to be used, no serious threat of deactivation due to excessive steam partial pressure, and no danger of forming a liquid under such conditions, the considerations relating to the problem of stripping and regenerating a FCC catalyst, then returning it to the FCC reactor, have little in common with those encountered in our process.
As one skilled in the art will also appreciate, a zeolite oligomerization catalyst must be regenerated at relatively low oxygen concentration to minimize catalyst deactivation, and regeneration is typically done by recirculating regenerator flue gas. To maintain required fluidization velocity through the regenerator, a high volumetric flow of oxygen-containing gas, whether air or flue gas, is also demanded. This demand cannot be met economically if regeneration is to be done at high pressure.
In each operation, namely stripping and regeneration, to maintain a sufficiently high volumetric flow (m.sup.3 /min, or ft.sup.3 /min) of stripping gas, an increase of operating pressure predetermines a correspondingly high mass flow (kilos/hr or pounds/hr) of gas. Aside from the fact that higher operating pressures require that stripping gas and regenerating gas be each compressed to the required high pressures at considerable cost for power, this second consideration of required volumetric flow, along with the first, relating to steam deactivation, determines that the efficacy of both stripping and regeneration will be adversely affected as pressure increases. Particularly as the cost of purchasing and operating a compressor tends to increase exponentially as its capacity and output pressure increases above about 100 psig, it is necessary, for economy, to operate with as low a pressure as will yield the desired process results.
Notwithstanding a desirably low superatmospheric operating pressure effective for the oligomerization reaction chosen, circulation of the catalyst through valves under conditions which dictate a substantial pressure drop, cause excessive attrition of the catalyst. It is fortuitous that the use of lock-hoppers permits control of the pressure drop from one stage to another such that deleterious attrition of the catalyst is minimized. The lock-hoppers also permit operation of the regenerator at as low a pressure as the other process conditions of its economical operation permit.
The unique feature of being able to operate this process with a low "coke-make" allows regenerating a very small fraction of the reactor inventory semicontinuously, which in turn may make a presently, otherwise typically uneconomical and unattractive process, attractive. This invention relates to a surprisingly effective and commercially significant solution to problems relating to stripping and regenerating the catalyst by adapting a system for "stepping down" the spent catalyst pressure to a low pressure, preferably about 50 psig, sufficient for adequate stripping, and then regenerating the catalyst at an even lower pressure.
Though for neither of the foregoing reasons, the concept of isolating the reactor and the regenerator from one another, was taught in U.S. Pat. No. 2,370,234 to Degnen et al (class 23/288) nearly fifty years ago, and in U.S. Pat. No. 2,854,161 to Payne (class 214/152); and, stepping down the reactor pressure for stripping and regenerating the catalyst has been used in numerous modifications of moving-bed processes. Continuous low pressure naphtha reforming is taught in U.S. Pat. No. 3,647,680 (class 208/65) and U.S. Pat. No. 3,752,348 (class 208/138 ) to Greenwood et al.; and U.S. Pat. No. 3,873,441 to Jones (class 208/166) teaches how to cope with the problems of withdrawing a mixed-liquid/gas-phase hydroprocessing catalyst from a high pressure reactor. More recently lock-hoppers have been used in moving bed catalytic reforming processes disclosed in U.S. Pat. No. 4,576,712 to Greenwood (class 208/138) and U.S. Pat. No. 4,615,792 (class 208/134). We know of no suggestion in the prior art to use a lock-hopper to transport spent zeolite catalyst from a fixed turbulent fluid bed reactor, to a fixed turbulent fluid bed regenerator via a pressure-isolatable zone and a stripper in which the catalyst could be effectively stripped.
None of the teachings of the foregoing references is reasonably applicable to solving the problems of our process because the problems we encounter arise under different technical circumstances. The solution to our problems is made possible only because of a peculiar set of circumstances. The zeolite catalyst used, such as a ZSM-5 type catalyst is especially well-adapted for use in high pressure reactors, but because of operation at such high pressure it has a high percent by weight (% by wt) of hydrocarbons in the voids and pores of the catalyst. Yet it should, and can be stripped quickly. If steam is chosen as the stripping medium, the catalyst can withstand exposure to steam provided the partial pressure of steam is not too high, because the higher the temperature and/or the partial pressure of steam, the greater the deactivation. Stripping and regeneration is quick because of the very low rate of coking of the catalyst, generally less than 1% by wt, and typically less than 0.4% by wt of the olefins charged, which low rate in turn permits a correspondingly low rate of withdrawal of the recycle stream. The unexpectedly low catalyst circulation rate makes the costs of depressuring/pressurizing affordable and ideally suited for this process.